It is desirable to provide a level of security in cable television transmissions so that the video (picture) and/or audio (sound) portions of a TV transmission may only be viewed and/or heard on a premium basis by subscribers who have paid a special fee (premium) therefor.
Techniques for scrambling the video portion of a television signal are generally well known. For instance, U.S. Pat. No. 4,024,575, entitled CATV SINE WAVE CODING SYSTEM and U.S. Pat. No. 4,112,464, entitled SUBSCRIPTION TV DECODER LOGIC SYSTEM disclose a scrambling format described wherein the video sync intervals are suppressed using a sinewave attenuation timing signal (sinewave scrambling signal). The sync is recovered by applying a reverse sinewave timing signal which is derived from the amplitude modulation on the FM sound carrier. The modulation of the sound carrier is normally 0.5 dB greater than that of the picture carrier.
Typical signal levels for the sinewave scrambling signal are 6.5 dB on the picture carrier and 7 dB on the sound carrier.
Commonly-owned U.S. Pat. Nos. 4,598,318 and 4,571,615 describe various techniques for scrambling television transmissions, and descrambling the transmissions at the subscriber (receiving) end of the network.
In other prior art sinewave descramblers, a feedback AGC loop detects the AM sinewave modulation on the sound carrier and applies it to attenuate the sinewave scrambling signal audio and video in a closed loop manner until about 0.5 dB of sinewave scrambling signal remains on the audio carrier. At that point, the sinewave scrambling signal on the picture carrier will be completely cancelled.
In all of the above techniques for descrambling, there is a tendency for noise from the sound carrier to find its way onto, and adversely affect the quality of, the picture carrier. This is especially noticeable, such as in streaking of the picture, when there is a poor carrier-to-noise ratio, such as on the order of 35 to 40 dB.